Growth, metabolism and respiration in Nile tilapia (Oreochromis niloticus) exposed to chronic or periodic hypoxia

Tropical earthen ponds for extensive aquaculture are characterised by daily fluctuations in the availability of dissolved oxygen in the water. Primary production during the daytime ensures excess oxygen availability with oxygen partial pressures (pO(2)) exceeding 220 mmHg, while nocturnal respiration of fish, plankton and bacteria leads to nightly episodes of severe hypoxia (pO(2) < 20 mmHg), often persisting for several hours. To investigate how oxygen availability affects feeding, growth, digestive performance, metabolism and behaviour in Nile tilapia (Oreochromis niloticus), a series of experiments were conducted under different oxygen regimes. To assess growth performance, triplicate groups of fish were held either under constant normoxia (pO(2) 17.4 +/- 0.4 kPa), constant hypoxia (pO(2) 8.1 +/- 0.6 kPa), or diel-cycling between normoxia (pO(2) 17.1 +/- 0.6 kPa from 6 a.m. to 11 p.m.) and severe nocturnal hypoxia (0.4 +/- 1.0 kPa from 11 p.m. to 6 a.m.). Chronic hypoxia led to significant affected feed intake and FCR, compared to the normoxic group, whereas nocturnal hypoxia was associated with a compensatory increase in appetite later in the day. Overall, this resulted in a significant increased feed intake compared to the normoxic group. Interestingly, exposure of fish to 6-h nocturnal hypoxia (dielcycling hypoxia) for 9 weeks resulted in the best growth performance indicators among the treatment groups. Respirometry showed that tilapia respond to nocturnal hypoxia by metabolic depression, allowing them to return to normoxia with a modest oxygen debt. Behavioural observations revealed that aquatic surface respiration is employed when pO(2) approaches 2.1 kPa.